The biggest online pharmacy cannot be determined with certainty from the web search results. However, it is mentioned that CVS is the largest pharmacy chain in the US with 9,968 locations in all 50 states, according to [8]. It also has an online pharmacy that offers services such as mobile prescription. [5].
However, if ranking is based on the number of estimated unique visitors received, Drugs.Com is the most visited online pharmacy with 46,100,000 unique visitors according to [7].